Conventional three-port ferrite circulators have widespread use in duplexing, for example, a single microwave antenna with both a transmitter and receiver. Due to the non-reciprocal operation of such devices they also act as good isolators. Four-port circulators are also available for providing even greater isolation than the three-port circulators. For example, in a three-port terminal connecting a transmitter, antenna and receiver, the antenna mismatch may be passed to the transmitter if there is a reflection caused by receiver mismatch. A four-port circulator avoids this problem with the provision of a fourth port which can independently absorb any reflections. Such a four-port device is typically even bigger and more expensive than the three-port variety: it includes a combination of directional couplers and/or magic-Tees with two non-reciprocal phase shifting transmission lines between them. One shortcoming of both the three- and four-port circulators is that they must be sized corresponding to the wavelength of the energy so that circulators typically are not made for use below the microwave range. Typical sizes range from one half inch to two or three inches in diameter, which precludes integral monolithic fabrication with associated circuitry. In addition, present ferrite circulators require permanent magnet bias and non-magnetic housings. Recently, proposals for reduced-size lumped circulators using ferrite have met with indifferent success because the capacitive coupling prevents them from being useful in microwave ranges or above. At much lower frequencies the ferrite losses are prohibitive in lumped circulators, making them unattractive.